Liguo Chi

Profibrillatory actions of pinacidil in a conscious canine model of sudden coronary death.

Pinacidil is one of a number of new antihypertensive agents possessing an action that involves an enhanced potassium efflux in cardiac and vascular smooth muscle. An associated feature of pinacidil is a shortening of the cardiac action potential duration, which may constitute a potentially proarrhythmic effect. The present study evaluated pinacidil (0.3 mg/kg/h i.v. for 6 h) on the postinfarcted canine heart in a subset of dogs unresponsive to programmed electrical stimulation during the subacute phase of anterior myocardial infarction, and known to be at low risk of ventricular fibrillation in response to acute posterolateral ischemia. Results were compared with a comparable control group of vehicle-treated, non-inducible animals. Nonsustained ventricular tachyarrhythmia developed in 2 of 15 pinacidil-treated animals as compared to the initiation of ventricular tachycardia in 1 of 16 postinfarcted hearts (p = 0.96) in the control group. Thus, pinacidil did not after the responsiveness of the postinfarcted heart with respect to the electrical induction of tachyarrhythmias. The subsequent development of an acute ischemic event at a site remote from the previous myocardial infarction was associated with a greater incidence of ventricular fibrillation within 1 h from the onset of ischemia in the pinacidil-treated animals (9/15: 60%) as compared to the control group (1/15: 6.7%; p = 0.007). The 24-h cumulative mortality, likewise, was greater in the pinacidil-treated group [13/15 (87%)] as compared to the vehicle-treated control group 3/15; 20%; p = 0.001. Significant cardiovascular and electrophysiologic effects of pinacidil included an increase in heart rate (124 ± 6–143 ± 10 beats/min, p < 0.05) and reductions in the refractory periods of normal (178 ± 2–166 ± 4 ms, p<0.05) and peri-infarcted (170 ± 5–185 ± 5 ms, p < 0.01) myocardial regions. It is concluded that pinacidil does not alter the responsiveness of the postinfarcted heart to programmed electrical stimulation. However, in the presence of a superimposed acute ischemic event, pinacidil increases the potential for the development of ventricular fibrillation in a subset of postinfarcted animals that otherwise show a low risk with respect to the development of lethal arrhythmias. It is hypothesized that the increased tendency to develop ventricular fibrillation is associated with the pinacidil-induced reduction in the ventricular refractory period. This conclusion is consistent with the known ability of pinacidil to enhance potassium efflux during myocardial repolarization and to decrease the duration of the action potential.

Superoxide dismutase conjugated to polyethylene glycol provides sustained protection against myocardial ischemia/reperfusion injury in canine heart.

Disagreement regarding the cardioprotective role of superoxide dismutase may relate to the use of different durations for induction of ischemic injury and reperfusion. The present study employed superoxide dismutase conjugated to polyethylene glycol (PEG-SOD), which has a half-life greater than 30 hours. Two protocols differing in the mode of administration and the duration of the reperfusion interval were used. Dogs were subjected to occlusion of the circumflex coronary artery for 90 minutes, then reperfused for 6 hours (Protocol A) or 4 days (Protocol B). The dogs received either polyethylene glycol conjugated to albumin (PEG-ALB) or PEG-SOD (1,000 U/kg). In Protocol A, treatment was administered starting 15 minutes before coronary occlusion and continued for 2 hours, terminating 15 minutes after reperfusion. Infarct size was determined 6 hours later. In Protocol B, the conjugated proteins were given 15 minutes before reperfusion and ended simultaneously with reperfusion. Infarct size was measured after 4 days. Infarct size (percentage of area at risk) in control (n = 9) and treated (n = 9) dogs in Protocol A differed between groups: 46.7±3.5% versus 28.3±2.9%, respectively (p±O.005); risk regions did not differ: 42.8±1.5% versus 43.8±2.1%, respectively. Myocardial salvage also was observed in Protocol B. Infarct size in control (n = 13) and treated (n = 13) groups was 44.2±2.6% versus 29.2±1.6%, respectively (p±0.005), with risk regions being 44.4±1.4% versus 46.0 ±1.6% (p = NS). Hemodynamic variables did not differ during the period of coronary artery occlusion. The respective collateral blood flows to the inner two thirds of the ischemic myocardium determined 60 minutes after occlusion were 0.05 ±0.01 ml/min/g and 0.06±0.04 ml/min/g (p = 0.806) for the PEG-ALB and PEG-SOD treated groups, respectively. Infarct size was related inversely to collateral blood flow in the PEG-ALB treated group. This relation shifted downward (analysis of covariance, p = 0.017). Plasma SOD activity in Protocols A sustained for 6 hours. Significant enzymatic activity was present after 4 days in Protocol B. Previous negative studies with native SOD may be related to the short half-life of its free-radical scavenging capacity, which compromises the chances of observing a protective effect after 4 days of reperfusion. The present results support our previous observations, as well as those of other investigators, demonstrating that superoxide dismutase can reduce that component of myocardial injury associated with reperfusion.

Heparin neutralization by platelet-rich thrombi. Role of platelet factor 4.

BACKGROUND Platelets contain several factors that inhibit heparin. This study was designed to assess the heparin-neutralizing activity present in acute, platelet-rich arterial thrombi formed at sites of arterial injury in animals. METHODS AND RESULTS Platelet-rich thrombi (n = 3) were induced in pig coronary arteries by balloon catheter-mediated arterial injury. Soluble extracts were prepared from each thrombus and assayed for the capacity to inhibit heparin in an in vitro clotting assay (activated partial thromboplastin time). Mean heparin-neutralizing activity was 28 U of heparin neutralized per milliliter of thrombus, indicating that 1 vol of coronary thrombus completely inhibited the heparin present in 140 vols of therapeutically anticoagulated (0.2 U heparin/mL) plasma. In contrast, thrombus extracts had no effect on the anticoagulant activity of hirudin, a direct-acting thrombin inhibitor. The heparin-neutralizing activity present in coronary thrombi bound to heparin-agarose and was eluted from it by 1.4 mol/L NaCl, suggesting that platelet factor 4 mediated the antiheparin effect of thrombi. Consistent with this hypothesis, a murine monoclonal antibody to rabbit platelet factor 4 nearly completely inhibited the heparin-neutralizing activity present in rabbit thrombi (n = 3) generated by carotid artery injury. CONCLUSIONS Extracts prepared from platelet-rich arterial thrombi significantly inhibit the in vitro anticoagulant potency of heparin but not of hirudin. This antiheparin effect appears to be mediated by platelet factor 4. These results are consistent with the hypothesis that localized inhibition of heparin at sites of platelet activation may reduce its antithrombotic efficacy. In addition, they suggest an additional mechanism for the apparent superiority of hirudin over heparin as a thrombin inhibitor at sites of arterial injury.

Actions of pinacidil at a reduced potassium concentration: A direct cardiac effect possibly involving the ATP-dependent potassium channel

We investigated the effects of the ATP-dependent K+ channel antagonist glyburide and the ATP-dependent K+ channel agonist pinacidil in a Langendorff-perfused rabbit isolated heart subjected to a period of global hypoxia. A class III antiarrhythmic drug, E-4031, also was studied in this model. These studies aimed to define the mechanism of action of putative profibrillatory actions of pinacidil and the mechanism for the antifibril-latory effect of the class III antiarrhythmic drug, E-4031, in the hypoxic heart. After stabilization and determination of baseline functional parameters under normoxic perfusion conditions (95% O2/5% CO2), hearts were subjected to global hypoxia by switching to a 95% N2/5% CO2 saturated perfusion medium for a period of 12 min. After the hypoxic period, normoxia was re-established by switching to the oxygen-carbon dioxide saturated buffer medium for a period of 40 min. The oxygen tension of the perfusion buffer was reduced from approximately 400 mm Hg to below 50 mm Hg during the hypoxic period. All hearts subjected to hypoxia had reduced function: the left ventricular developed pressure and ±dPldt were reduced significantly. Myocardial tissue ATP concentrations were reduced (>50%) in hearts subjected to hypoxia. Under conditions of hypoxic/reoxygenation and in the presence of a low (2.5 mM) potassium concentration ([K+]0), pinacidil (1.25 μM) facilitated the induction of ventricular fibrillation (80% fibrillation in the presence of pinacidil vs. 20% in the absence of pinacidil). Glyburide (10 μM) and E-4031 (1 and 10 μM) significantly reduced the incidence of ventricular fibrillation associated with pinacidil (20% fibrillation in the presence of hypoxia, pinacidil, and glyburide or 10 μM E-4031). Opening of the ATP-dependent K + channel by pinacidil under normoxia and low K + also facilitated the induction of ventricular fibrillation (60% ventricular fibrillation). Pinacidil failed to induce ventricular fibrillation under either normoxic or conditions of hypoxic/reoxygenation when the [K + ]0 was increased to 5.1 mM. The results of this study demonstrate that K + channel activators facilitate the induction of ventricular fibrillation under both normoxic conditions and conditions of hypoxic/reoxygenation when the perfusion buffer K+ concentration is reduced.

Protective Effects of Ranolazine on Ventricular Fibrillation Induced by Activation of the ATP-Dependent Potassium Channel in the Rabbit Heart.

Background: The authors studied the antifibrillatory effects of the adenosine-triphosphate (ATP)-sparing metabolic modulator ranolazine in a rabbit isolated heart model in which ventricular fibrillation occurs under conditions of hypoxia/reoxygenation in the presence of the ATP-dependent potassium channel opener pinacidil. Methods and Results: Ten minutes after ranolazine or vehicle administration, addition of pinacidil (1.25 μM) to the buffer was followed by a 12-minute hypoxic period and 40 minutes of reoxygenation. At a reduced concentration of ranolazine (10 μM), ventricular fibrillation occurred in 60% of the hearts. compared to 89% in the control group (P = NS). In contrast, only three of nine hearts (33%) treated with 20 μM ranolazine developed ventricular fibrillation (P <.05 vs vehicle). Hemodynamic parameters including coronary perfusion pressure, left ventricular developed pressure, and ±dP/dt were not affected by the presence of ranolazine in the perfusion medium. Ranolazine did not prevent or modify the negative inotropic or coronary vasodilator actions of pinacidil, suggesting a mechanism of action independent of potassium channel antagonism. Conclusions: Ranolazine significantly reduced the incidence of ventricular fibrillation in the hypoxic/reoxygenated heart exposed to the ATP-dependent potassium channel opener, pinacidil. The reported ability of ranolazine to prevent the decrease in cellular ATP during periods of a reduced oxygen supply may account for its observed antifibrillatory action. By maintaining intracellular ATP, ranolazine may modulate or prevent further opening of the ATP-dependent potassium channel in response to hypoxia and/or pinacidil.

Effects of tedisamil (KC-8857) on cardiac electrophysiology and ventricular fibrillation in the rabbit isolated heart.

1 The direct cardiac electrophysiological and antifibrillatory actions of tedisamil (KC‐8857) were studied in rabbit isolated hearts. 2 Tedisamil (1, 3, and 10 μm), prolonged the ventricular effective refractory period (VRP) from 120±18ms (baseline) to 155±19, 171±20, and 205±14 ms, respectively. Three groups of isolated hearts (n = 6 each) were used to test the antifibrillatory action of tedisamil. Hearts were perfused with 1.25 μm pinacidil, a KATP channel activator. Hearts were subjected to hypoxia for 12 min followed by 40 min of reoxygenation. Ventricular fibrillation (VF) developed during hypoxia and reoxygenation in both the control and 1 μm tedisamil‐treated groups (5/6 and 4/6, respectively). Tedisamil (3 μm) reduced the incidence of VF (0/6, P = 0.007 vs. control). 3 In a separate group of hearts, VF was initiated by electrical stimulation. The administration of 0.3 ml of 10 mM tedisamil, via the aortic cannula, terminated VF in all hearts, converting them to normal sinus rhythm. 4 Tedisamil (3 μm) reversed pinacidil‐induced negative inotropic effects in rabbit isolated atrial muscle which were equilibrated under normoxia, as well as in atrial muscle subjected to hypoxia and reoxygenation. 5 The results demonstrate a direct antifibrillatory action of tedisamil in vitro. The mechanism responsible for the observed effects may involve modulation by tedisamil of the cardiac ATP‐regulated potassium channel, in addition to its antagonism of IK and Ito.

Antiarrhythmic and electrophysiologic actions of CK-3579 and sematilide in a conscious canine model of sudden coronary death.

The antiarrhythmic and antifibrillatory actions of the CK-3579 and sematilide, two new class III antiarrhythmic drugs, administered in a multiple-dose regimen were evaluated in conscious dogs 3–5 days after anterior myocardial infarction. The study population consisted of three groups of 10 dogs each, in which all animals entered into the final protocol developed nonsustained or sustained ventricular tachycardia in response to programmed electrical stimulation using one, two or three premature stimuli. Each drug was administered intravenously in a dose of 3.0 mg/kg every 3 h for a total of six doses. Sematilide significantly suppressed the induction of ventricular tachyarrhythmia by programmed electrical stimulation in six of 10 postinfarcted dogs, whereas CK-3579 suppressed the induction of tachyarrhythmia in only two of 10 animals. Despite its ineffectiveness in preventing electrical induction of tachycardia, CK-3579 produced a significant increase in the cycle length of the induced ventricular rhythm. The administration of each drug was associated with an increase in the ventricular refractoriness and in the paced QT interval, suggesting that class III electrophysiologic properties contribute to the antiarrhythmic action of each drug. In addition, CK-3579 was shown to have β1-adrenoceptor blocking properties. The subsequent induction of an acute ischemic event in a region remote from the infarct-related artery was associated with a high incidence (80%, eight of 10 postinfarcted dogs) of ventricular fibrillation within the first hour after the onset of myocardial ischemia in the vehicle-treated control group. Both CK-3579 and sematilide provided a significant degree of protection (80 and 70% survival, respectively) against the development of sudden death due to ventricular fibrillation, as well as cumulative mortality determined at 24 h. The data suggest that both CK-3579 and sematilide induce cardiac electrophysiologic changes characteristic of drugs which are classified as class III antiarrhythmic agents. The most striking property of each drug appears to be related to the prevention of ventricular fibrillation as assessed in an experimental model of sudden death associated with the superimposition of an ischemic event in a region remote from a previous myocardial infarct.

MS-551 protects against ventricular fibrillation in a chronic canine model of sudden cardiac death.

Summary We studied the electrophysiologic and anti-fibrillatory properties of MS-551 (l,3-dimethyl-6-{(2-[N-hydroxy-ethyl)-3-(4-nitrophenyl) propylamino] ethy-lamino} 2,4(1H,3H) pyrimidinedione hydrochloride) in a conscious canine model of sudden cardiac death. Three to 5 days after surgically induced myocardial infarction (MI: 2-h occlusion of the left anterior descending coronary artery, LAD), animals were subjected to programmed electrical stimulation (PES) to identify those at risk for sudden cardiac death. MS-551 was administered (2.0, 3.0, or 4 ± 2.0 mg/kg intravenously, i.v.). Vehicle-treated animals received 0.9% sodium chloride solution for injection. MS-551 (multiple-dose regimen) increased ventricular effective refractory period (VERP) from 112 ± 4 to 137 ± 4 ms (p > 0.05) as compared with vehicle treatment, which did not alter VERP (125 ± 6 to 121 ± 5 ms). MS-551 prolonged QTc interval from a predrug value of 293 ± 8 to 333 ± 18 ms postdrug. The size of surgically induced MI did not differ among groups: 2.0 mg/kg, 23 ± 4%; 3.0 mg/kg, 28 ± 2%; 4 ± 2.0 mg/kg, 25 ± 3%; and vehicle, 28 ± 3% of the left ventricle. Single bolus administration of MS-551 (2.0 or 3.0 mg/kg i.v.) did not confer significant protection against sudden cardiac death. However, repeated administration of MS-551 protected against sudden cardiac death in 8 of 10 dogs as compared with 2 of 12 in the vehicle-treated group (p > 0.05). The data indicate that a multiple-dose regimen of MS-551 provides protection against ischemia-induced ventricular fibrillation (VF) in the postinfarcted heart. The mechanism by which MS-551 achieves its antifibrillatory effect most likely depends on its ability to prolong VERP of myocardium without altering ventricular conduction velocity.

Electrophysiology and Antiarrhythmic Actions of E-4031 in the Experimental Animal Model of Sudden Coronary Death

The Class III agent E-4031 was evaluated for its antiarrhythmic and antifibrillatory actions in conscious dogs 3–5 days after anterior myocardial infarction that were responsive to the induction of tachyarrhythmia by programmed electrical stimulation. The administration of E-4031 as an intravenous loading dose (100 μg/kg) followed by an infusion for 90 min (10 μg/kg/min) suppressed the induction of ventricular tachycardia by programmed electrical stimulation in 6 of 12 dogs and prolonged the cycle length of the induced arrhythmia in 5 of the 6 remaining animals. Continued administration of E-4031 in a dose regimen of 1,000 μg/kg every 2 h provided significant protection (8 of 10 dogs) against the development of ventricular fibrillation (sudden coronary death) within the first hour after the onset of myocardial ischemia in a region of the ventricle remote from the infarctrelated vessel. The incidence of sudden coronary death was 80% in a comparable control group of electrically inducible postinfarcted dogs. Increases in ventricular myocardial refractoriness in the paced QT and QTc intervals suggest that Class III electrophysiologic actions contribute to the antiarrhythmic and antifibrillatory actions of E-4031. The findings suggest that E-4031 may be of clinical utility in the prevention of life-threatening arrhythmias in the setting of myocardial ischemia in the postinfarcted heart.

Antiarrhythmic versus antifibrillatory actions: Inference from experimental studies

Pathophysiology of the coronary circulation is a major contributor to altering the myocardial substrate, rendering the heart susceptible to the onset of arrhythmias associated with sudden cardiac death. Antiarrhythmic drug therapy for the prevention of sudden cardiac death has been provided primarily on the basis of trial and error and in some instances based on ill-suited preclinical evaluations. The findings of the Cardiac Arrhythmia Suppression Trial (CAST) requires a reexamination of the manner in which antiarrhythmic drugs are developed before entering into clinical testing. The major deficiency in this area of experimental investigation has been the lack of animal models that would permit preclinical studies to identify potentially useful or deleterious therapeutic agents. Further, CAST has emphasized the need to distinguish between pharmacologic interventions that suppresses nonlethal disturbances of cardiac rhythm as opposed to those agents capable of preventing lethal ventricular tachycardia or ventricular fibrillation. Preclinical models for the testing of antifibrillatory agents must consider the fact that the superimposition of transient ischemic events on an underlying pathophysiologic substrate makes the heart susceptible to lethal arrhythmias. Proarrhythmic events, not observed in the normal heart, may become manifest only when the myocardial substrate has been altered. We describe a model of sudden cardiac death that may more closely simulate the clinical state in humans who are at risk. The experimental results show a good correlation with clinical data regarding agents known to reduce the incidence of lethal arrhythmias as well as those showing proarrhythmic actions.